Backspace is indeed the most useful, but others are too, especially when using the terminal. For example, we don't have an email client on touch so I use mutt in a terminal. I need to use h, j, k and l to move around. If I have 200 messages, I have to tap 'j' 200 hundred times to go to the bottom of the list of emails. I think a double-tap hold might work well, but I'll leave that to others to decide.

I think the issue here with auto-repeat is that press and hold currently invoke the popup menu with additional characters (for some keys). We'd need a mechanism to invoke the auto-repeat. Maybe press and hold for a certain amount of time would invoke auto-repeat. Needs some guidance from design here.